Anthrax is a potentially lethal human infection. The causative organism is a Gram-positive rod-shaped bacterium, Bacillus anthracis, which exists in a vegetative or a spore form. Spores are the infecting agent; infection is initiated by entry of spores into a mammalian host. Entry can be by intradermal inoculation, ingestion, or inhalation. The most lethal form of anthrax in humans is pulmonary infection caused by inhalation of B. anthracis spores.
Like all Bacillus species, B. anthracis bacteria form spores when subjected to adverse conditions. Mature spores are dormant and highly resistant to heat, dryness, and aggressive chemical conditions. They can survive in soil for decades. Upon entry into a suitable host, the spores germinate and multiply rapidly. The bacteria then release the anthrax toxins toxic to the host.
Most Bacillus spores consist of a central genome-containing core surrounded by two protective layers: the cortex and the coat. The outer layer of most Bacillus spores is the spore coat comprised of different proteins. Mature spores of Bacillus species such as B. anthracis contain an additional loose-fitting layer called an exosporium. The exosporium is the outermost layer for B. anthracis and interacts with the environment/host. The exosporium is the primary permeability barrier of the spore and contains spore surface antigens.
Analysis of the exosporium identified several protein including a glycoprotein called BclA (Bacillus collagen-like protein of anthracis). BclA is a structural component and contains multiple collagen-like Xaa-Yaa-Gly repeats. BclA is an immuno-dominant protein on the B. anthracis spore surface because most of the antibodies raised against spores react with this protein.
An unusual tetrasaccharide is attached to the BclA protein, likely through a GalNAc linkage. This tetrasaccharide consists of three rhamnose monosaccharides linked to a sugar residue called anthrose [2-O-methyl-4-(3-hydroxy-3-methylbutanamido)-4,6-dideoxy-β-D-glucose]. Anthrose was reported to be unique to B. anthracis spores, however the anthrose biosynthesis genes were recently identified also in other bacilli and it was demonstrated that anthrose expression is not restricted to B. anthracis. 